• 0.9 baseline — 0.6 × season 0.9 (11 gm) + 0.4 × last 10 0.8
• +0.1 opponent leak — they concede 4.5/game vs a 4.2 league average
• −0.5 at this ground — averages 0.4 there (5 gm)
• −0.1 wet forecast — averages 0.8 in the wet (8 gm)
• vs them: averages 0.7 over 10 games — evidence, never summed

Derived, not measured — plain arithmetic over his real games, shown in full. Adjustments can overlap (a game at this ground can also be a wet one); they add simply, and this note is the admission.